Typically, variable gain amplifiers fall into one of two categories: multiplier based or digitally controlled. The multiplier based amplifiers are usually based on Gilbert cells and can provide continuous variability in terms of gain. Digitally controlled amplifiers, on the other hand, provide discrete gains, which may not be suitable for many applications. With conventional continuously variable amplifiers, such as Gilbert cell type amplifiers, step transitions (such as “on” and “off”) can generate distortion, such as a “pop” or “click” with audio applications. Additionally, Gilbert cell multipliers would generally require high performance bipolar transistors, which are not generally available in CMOS processes.
Turning to FIG. 1, an example of conventional continuously variable amplifier 100 can be seen. Amplifier 106 generally comprises a transconductance circuit 104, a feedback network (i.e., resistors R5 ad R6), and a shunt circuit 106. The shunt circuit 106 generally comprises resistors R1 through R4, and shunt switches Q1 and Q2 (which can be NMOS transistors as shown). In operation, the gain control signal GC is applied to the shunt switches Q1 and Q2. As the gain control signal GC is ramped or increased from 0V, the shunt switches Q1 and Q2 begin shorting the differential input signal applied to the shunt circuit 106 by the input source 102 so as to mute or “shut off” the input signal. Because this shunt circuit 106 is in the main signal path (from the input source 102 to the transconductance circuit 104), the shunt circuit 106 can introduce distortion. Point in fact, as the gain control signal GC increases, the total harmonic distortion (THD) increases, meaning that the THD performance of amplifier 100 limits the gain range of amplifier 100. Therefore, there is a need for an improved amplifier.
Some other conventional circuits are: U.S. Pat. Nos. 5,436,588; 5,537,081; 6,774,684; and 7,227,413.